EtherChannel

EtherChannel

EtherChannel is a port trunking (aggregation in Cisco's term) technology primarily used on Cisco switches. It allows grouping several physical Ethernet links to create one logical Ethernet link for the purpose of providing fault-tolerance and high-speed links between switches, routers and servers. An EtherChannel can be created from between two and eight active Fast Ethernet, Gigabit Ethernet or 10 Gigabit Ethernet ports, with an additional one to eight inactive/failover ports which becomes active as the other active ports fail. EtherChannel is primarily used in the backbone network, but can also be used to connect UNIX and PC Servers. A limitation of EtherChannel is that all the physical ports in the aggregation group must reside on the same switch. The SMLT protocol removes this limitation by allowing the physical ports to be split between two switches.

EtherChannel technology was invented by Kalpana in the early 1990s. They were later acquired by Cisco Systems in 1994. In 2000 the IEEE passed 802.3ad which is an open standard version of EtherChannel.

Benefits

Using an EtherChannel has numerous advantages, and probably the most desirable aspect is the bandwidth. Using the maximum of 8 active ports a total bandwidth of 800 Mbit/s, 8 Gbit/s or 80 Gbit/s is possible depending on port speed. This assumes there is a traffic mixture, as those speeds do not apply to a single application only. It can be used with Ethernet running on unshielded twisted pair (UTP) wiring, single-mode and multimode fiber.

Because EtherChannel takes advantage of existing wiring it makes it very scalable. It can be used at all levels of the network to create higher bandwidth links as the traffic needs of the network increase. All Cisco switches have the ability to support EtherChannel.

When an EtherChannel is configured all adapters that are part of the channel share the same Layer 2 (MAC) address and Layer 3 (IP) Address. This makes the EtherChannel transparent to network applications and users because they only see the one logical connection; they have no knowledge of the individual links.

EtherChannel aggregates the traffic across all the available active ports in the channel. The port is selected using a Cisco-proprietary hash algorithm. These load-sharing algorithms vary between platforms due to the fact that decisions are based on source or destination Media Access Control (MAC) Addresses, IP addresses or TCP/UDP Port numbers. The following table illustrates the ratios at which EtherChannel balances the load across the ports.

Number of Ports Load Balancing
8 1:1:1:1:1:1:1:1
7 2:1:1:1:1:1:1
6 2:2:1:1:1:1
5 2:2:2:1:1
4 2:2:2:2
3 3:3:2
2 4:4

Fault-tolerance is another key aspect of EtherChannel. Should a link fail, the EtherChannel technology will automatically redistribute traffic across the remaining links. This automatic recovery takes less than one second and is transparent to network applications and the end user. This makes it very resilient and desirable for mission-critical applications.

Spanning Tree Protocol can be used with an EtherChannel. STP treats all the links as one and Bridge Protocol Data Units, (BPDU), are only sent down one of the links. Without the use of an EtherChannel STP would effectively shutdown any redundant links between switches until one connection goes down. This is where an EtherChannel is most desirable, it allows full use of all available links between two devices.

EtherChannels can be also configured as VLAN trunks. If any one port of an EtherChannel is configured as a VLAN Trunk, the entire EtherChannel will act as a VLAN trunk. Cisco Inter-Switch Link (ISL) VLAN Trunking Protocol (VTP) as well as 802.1q are compatible with EtherChannel

Components

EtherChannel is made up of the following key elements:

Ethernet Links - EtherChannel works over Industry-Standard 802.3 Ethernet Links, at all Ethernet speeds. All ports in a single EtherChannel must be the same speed. Multiple Etherchannels per device are supported, the number depends on the type of equipment. Catalyst 6500/6000 switches support a maximum of 64 EtherChannels.

Compatible Hardware - The entire line of Cisco Catalyst Switches as well as Cisco IOS software-based routers support EtherChannel. Configuring an EtherChannel between a switch and a PC or UNIX computer would require special network interface cards such as the Intel card pictured here

Configuration - An EtherChannel must be configured using the Cisco IOS on switches and router, and using specific drivers when connecting a server. There are two main ways an EtherChannel can be setup. The first is by manually issuing a command on each port of the device that is part of the EtherChannel. This must be done for the corresponding ports on both sides of the EtherChannel. The second way is using Port Aggregation Protocol (PAgP), which is a Cisco Systems proprietary networking protocol. PAgP is used for the automated aggregation of Ethernet ports into an EtherChannel.

EtherChannel vs. 802.3ad

EtherChannel and 802.3ad are very similar and accomplish the same goal. There are a few differences between the two other than the fact that EtherChannel is Cisco proprietary and 802.3ad is an open standard.

EtherChannel 802.3ad
Requires switch configuration. Little, if any, configuration of switch required to form aggregation. Some initial setup of the switch may be required.
Supports different packet distribution modes. Supports only standard distribution mode.

Both technologies are capable of automatically configuring this logical link. EtherChannel uses Port Aggregation Protocol (PAgP) which is a Cisco Systems proprietary networking protocol, whereas 802.3ad uses Link Aggregation Control Protocol (LACP), an open standards based protocol.

References

See also

External links

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